Method of fabricating miniature bellows by electroless chemical deposition



June 26, 1962 v. E. HAMREN 3,040,426

METHOD OF FABRICATING MINIATURE BELLOWS BY ELECTROLESS CHEMICAL DEPOSITION Filed Oct. so, 1958 CHEM/64L L Y DEPDJ/ TED ,4/1/0 .Sl/BSEQUE/VTLY HEAT- TREA TED NICKEL INVENTOR. 1 /6 rm 5 AQMRE/V ATTORNEY 3,040,426 METHOD OF FABRICATING MINIATURE BEL- BY ELECTROLESS CHEMICAL DEPOSI- This invention relates to miniature pressure sensing elements such as bellows, capsules, diaphragms, Bourdon tubes, functioning to provide mechanical movement in response to pressure changes. The invention further relates to an improved process of fabricating such miniature pressure sensing elements having wall structure of such thinness as to be Within the range of film thickness (e.g. roughly in a range which may include the range of .0002. inch up to .005 inch) and wherein the physical dimension such as diameter and/or length may be in a range extending down to as small as inch or less for axial length dimension and inch or less for outer diameter. It will be obvious that with such exceedingly small proportions, unusual problems of fabrication will arise, and a basic object of this invention is to provide an improved process of fabricating such miniature pressure sensing elements satisfactorily to attain a product of satisfactory durability, long life, resistance to temperature change, vibration and shock, and having a good spring rate.

A particular object of the invention is to provide such a miniature pressure sensing element which is highly resilient and provides a very low spring rate, approaching a Zero value, whereby to provide maximum accuracy in response to pressure changes over a satisfactory range of deflection of the element.

A further object of the invention is to provide such a miniature pressure sensing element having a uniform wall thickness throughout all areas whether fiat or curved or sharply bent. Another object is to provide the characteristics derived from this uniform wall thickness, namely an extremely low spring rate combined with relatively high strength. i

- Another object is to provide such a pressure sensing element embodying a combination of low spring rate, high ratio of deflection range to free length, and low hysteresis, i.e. the ability to faithfully return to a normal configuration after having been deflected therefrom.

In general, the invention contemplates a method of fabricating a miniature pressure sensing element such as a bellows by chemical deposition of a metal such as nickel upon a pattern or form in a manner to produce a coating thereon of uniform thickness, then removing the form by etching or chemical dissolution or by melting, leaving the film in the form of a hollow body of the desired configuration.

Hitherto, bellows and similar pressure sensitive flexible devices have generally been fabricated by rolling and hydraulically forming from tubes into convoluted shapes, from material such as brass, beryllium copper, stainless steel etc; or by assembling and joining the margins of stamped sheet metal washers, or by the molding of plas tics. Such earlier processes have the following disadvantages which are particularly acute where miniature bellows are concerned: rolling or hydraulic forming may be used only with materials having sufficient stretch to avoid break-down when stretched. Even then, intermittent annealing is required between stages of rolling or stretching. Cross sectional configuration is limited to those designs which provide access for the tooling. Such methods cannot be used for fabricating miniature parts. Very uneven wall thicknesses result from the stretching 3,040,426 Patented June 26, 1962 and bending operations, and engineering is thereby greatly complicated. Very light spring rates are not possible in such devices. Hysteresis is high and therefore the range of deflection is limited.

In the welded seam type, high cost of fabrication is entailed. Miniaturization is limited. Initial tooling cost is very high. The multiple joint between the parts complicates the hysteresis problem. Deflection is limited because of the over-stressing of the joints in any attempted high deflection operation.

Electro-plated parts are objectionable in that very uneven wall thicknesses result from the variable distance between the electrodes. Such parts are low in yield, ultimate strength is also low, and they are not sufliciently flexible. Undesirable magnetic properties are encountered in such parts.

Molded plastic bellows are not satisfactory for most purposes because of low strength, porosity, high hysteresis, and temperature limitations (they break down at high temperatures and become stifl at low temperatures) and they engender attachment problems.

Other objects and advatages will become apparent in the ensuing specification and appended drawing in which:

The single FIGURE discloses a cross sectional view through a bellows embodying the invention.

Referring now to the drawing in detail, I have shown therein, as an example of one form of pressure sensing element in which the invention may be embodied, a bellows which, though shown on enlarged scale, may be of quite small proportions, e.g. in the neighborhood of a quarter inch outer diameter, and having an extremely thin wall which may be as small as a range between .0002 inch and .005 inch, and the invention is also useful throughout the full range from this minimum range up to .005 inch thickness. Such a bellows may be called for in any one of a number of lightweight sensitive instruments such as are used in missiles, and it will be immediately apparent that the ordinary bellows fabricating techniques will not be satisfactory for fabricating such a small part.

The bellows is of a nickel-phosphorous alloy and has a substantially zero spring rate combined with relatively high strength, a low hysteresis, and a high ratio of deflection range to free length. Its wall thickness is uniform throughout all areas, whether flat or curved and regardless of the radius of curvature. The bellows has no inherent stresses, and has a maximum fidelity of linear response to pressure changes acting thereon.

Notwithstanding the fact that the film is a deposited film, which tends to exhibit a high yield with respect to ultimate strength, the bellows wall is exceedingly tough and flexible and resistant to failure throughout repeated flexings.

The bellows is formed by a process wherein a highly accurate pattern is first produced (as by die casting or molding).

The patterns are of aluminum or other etchable or thermo disintegrating material capable of being treated for satisfactorily receiving chemical nickel plating on the surfaces thereof. The forms are fabricated and then treated for plating receptivity, and are then chemically cleaned, using methods customary in the plating industry. The form is then dipped into a plating solution including an aqueous solution of a nickel salt and a hypophosphite, such as for example, a mixture of nickel sulfate and sodium hypo-phosphite, maintained at a temperature of to degrees centigrade and a pH of 4.0-6.0. The wall thickness is controlled by the length of time the form is maintained in the solution. After the determined thickness of film has been deposited, the pattern with the film adhering thereto is removed from the plating solution and washed, and is then subjected to heat treating at approximately 350 F. for normalizing and developing maximum properties therein.

Subsequently, the form is removed from the plated shell by an etching process (or by heating in the event the form material is one that can be melted at a temperature below the critical temperature at which the film would be melted or damaged). Access to the form may be provided by the opening formed by the stem upon which the form is suspended in the solution.

After the form has been removed, the shell is chemically cleaned both internally and externally and in some instances is covered by a light coating of gold plating. Trimming operations may then be performed, and headers may then be attached to the bellows over the open ends thereof after heat forming, if this is desired.

For special applications, the shell is then subjected to a heat forming step including heat treatment at approximately 425' F. for a time which may be approximately one hour. This is a heat forming operation during which the bellows is axially stressed, either in compression or extension, to the axial dimension desired. More broadly stated, the pressure sensing element (e.g. including a Bourdon tube) is stressed along the path of its pressureresponsive motion in its subsequent operation.

In some of its forms, the invention may provide a bellows which has been longitudinally deformed (either by compressing it or by extending it axially) to a limit position at one end or the other at the range of deflection of the bellows, and then setting the bellows in this limit position by heat treating it for an hour at approximately 425 F. The heat treatment will eliminate all residual strains and stresses in the wall of the bellows, leaving it in the deformed condition either at the beginning or end of its range of deflection, and thus the bellows may be adapted for a full range of deflecting movement entirely on one side of its normal free (unstressed) position. For example, the bellows may be fully compressed in its normal unstressed state and be adapted to extend through a full range of deflection to a maximum length, with all of such deflection being on one side of the free position of the bellows. Vice versa, the bellows may be pre-forrned in a position of maximum extension and from that position its entire range of movement may be toward the position of maximum compression.

I have discovered that it is fully feasible to perform this step on a bellows having a wall as thin as two ten thousandths of an inch, without damaging the bellows, and that bellows with improved properties (e.g. greatly increased magnitude of deflection in one direction, in proportion to free length) may be attained.

' I claim:

1. A method of fabricating a miniature fluid pressure sensitive convoluted bellows throughout a range of sizes extending downwardly to a lower limit of less than inch diameter, the steps of said method comprising: preparing a form in the approximate shape of the internal wall of the bellows; thoroughly cleaning the form; treating the surface of the form for acceptance of chemical plating; immersing the form in a solution of nickel and phosphorous salts and chemically depositing on the form a shell having the full diameter of the finished article and consisting of a film of uniform thickness in the range of .0002 and .005 inch and continuing said immersion until the shell attains self-sustaining condition; etching the form out of the shell after the shell has attained said self-sustaining condition; heat treating the shell at approximately 350 to relieve all internal stresses in the shell; chemically cleaning the shell; trimming the shell; and then subjecting the shell to a second heat treatment at approximately 425 F. while deforming the shell along its deflection axis to a selected shape to be established as its free shape, and continuing said second heat treatment until the shell assumes said deformed shape as its free shape.

2. The method defined in claim 1, wherein the second heat treatment and deforming step is carried on for approximately one hour.

3. A method of fabricating miniature fluid pressuresensitive convoluted bellows throughout a range of sizes extending downwardly to a lower limit of less than 4; inch diameter, the steps of said method comprising: preparing a form having substantially the shape of the internal wall of the bellows and being of the full diameter of the finished bellows; treating the surface of the form for acceptance of chemical plating; immersing the form in a solution of nickel and phosphorus salts and thereby chemically depositing thereon a shell consisting of a film of uniform thickness in the range between .0002 and .005 inch and continuing said immersion until the shell attains self-sustaining condition; subsequently heat-treating the shell for stress relief; etching the form out of the shell after the shell attained said self-sustaining condition; and then preparing the shell for use without any substantial change in diameter thereof.

4. A method of fabricating miniature fluid pressuresensitive convoluted bellows throughout a range of sizes extending downwardly to a lower limit of less than Ms" diameter, the steps of said method comprising: preparing a form having substantially the shape of the internal wall of the bellows and being of the full diameter of the finished bellows; treating the surface of the form for acceptance of chemical plating; immersing the form in a solution of nickel and phosphorus salts and thereby chemically depositing thereon a shell consisting of a film of uniform thickness in the range between .0002 and .005 inch and continuing said immersion until the shell attains self-sustaining condition; etching the form out of the shell after the shell has attained said self-sustaining condition; and then subjecting the shell to finish-processing without any substantial change in its diameter.

References Cited in the file of this patent UNITED STATES PATENTS 1,368,253 Fulton Feb. 15, 1921 1,648,046 Fulton Nov. '8, 1927 1,886,803 Giesler Nov. 8, 1932 2,534,124 Hasselhorn Dec. 12, 1950 2,833,029 Kearns May 6, 1958 2,841,866 Schilling July 8, 1958 2,865,375 Banks et al. Dec. 23, 1958 

1. A METHOD OF FABRICATING A MINIATURE FLUID PRESSURE SENSITIVE CONVOLUTED BELLOWS THROUGHOUT A RANGE OF SIZES EXTENDING DOWNWARDLY TO A LOWER LIMIT OF LESS THAN 1/8 INCH DIAMETER, THE STEPS OF SAID METHOD COMPRISING: PREPARING A FORM IN THE APPROXIMATE SHAPE OF THE INTERNAL WALL OF THE BELLOWS; THOROUGHLY CLEANING THE FORM; TREATING THE SURFACES OF THE FORM IN A SOLUTION OF NICKEL AND PLATING; IMMERSING THE FORM IN A SOLUTION OF NICKEL AND PHOSPHOROUS SALTS AND CHEMICALLY DEPOSITING ON THE FORM A SHELL HAVING THE FULL DIAMETER OF THE FINISHED ARTICLE AND CONSISTING OF A FILM OF UNIFORM THICKNESS IN THE RANGE OF .0002 AND .005 INCH AND CONTINUING SAID IMMERSION UNTIL THE SHELL ATTAINS SELF-SUSTAINING CONDITION; ETCHING THE FORM OUT OF THE SHELL AFTER THE SHELL HAS ATTAINED SAID SELF-SUSTAINING CONDITION; HEAT TREATING THE SHELL AT APPROXIMATELY 350* TO RELIEVE ALL INTERNAL STRESSES IN THE SHELL; CHEMICALLY CLEANING THE SHELL; TRIMMING THE SHELL; AND THEN SUBJECTING THE SHELL TO A SECOND HEAT TREATMENT AT APPROXIMATELY 425*F. WHILE DEFORMING THE SHELL ALONG ITS DEFLECTION AXIS TO A SELECTED SHAPE TO BE ESTABLISHED AS ITS FREE SHAPE, AND CONTINUING SAID SECOND HEAT TREATMENT UNTIL THE SHELL ASSUMES SAID DEFORMED SHAPE AS ITS FREE SHAPE. 